Samples Containing Mercury

General Information

Occurrence — Mercury is not abundant, nor widely distributed. Its abundance in the earth's crust is 0.085 ppm. It is found in the upper portions of cinnabar deposits, a mineral HgS, the chief source of the element. Hg has been found in quartz, sandstone, schists, iron pyrites, bituminous substances, eruptive, and sedimentary rocks of all ages. It occurs as the chloride in horn silver. It is occasionally associated with Zn ores and is generally found locally concentrated. The minerals that are more common are cinnabar, HgS; calomel, Hg₂Cl₂; coloradoite, HgTe; amalgam, Ag·Hg; livingstonite, HgSb₄O₇; and tiemannite, HgSe.

Uses — Metallic mercury dates back to 1600 B.C. Aristotle called it "fluid silver." Hg was used in the application of gold leaf (gilding). The sulfide was used as a pigment. Current uses of Hg include thermometers and barometers, gas containment, vacuum pumps, Hg vapor lamps, in amalgams with metals like Ag, Sn, and Na (dentistry, reducing agents, Au extraction from ores), treatment of skin diseases, antiseptics, fungicides, and related agents. Metallic mercury is an excellent electrode material in electrochemistry. The overpotential for H₂↑ evolution is high, extending the potential usable range by over 1.0 volts vs. SCE over a Pt electrode. It is used as a coating on solid electrodes, such as Pt, or as a Dropping Mercury Electrode (DME), first used in controlled potential voltammetry in the 1920s.

Hg Chemistry as Practiced & Observed at IV

The Group 12 elements do not form ions with incomplete d shells (their electronic structure is (period-1)d¹⁰period s²). Unlike Zn and Cd, which only have the +2 oxidation state due to the stability of the filled d orbital, Hg has been assigned the +1 as well as the +2 oxidation state. However, due to its electronic structure [Hg(5d¹⁰6s²) → Hg²⁺(5d¹⁰)] and the stability of the filled 5d¹⁰, it appears that Hg⁺ either does not exist, or only exists as a diatom (Hg:Hg)²⁺, which contains :Hg⁰ and Hg²⁺ rather than two ·Hg⁺ atoms (here the 6s² electrons are represented as a colon ":" and the 6s¹ electron as "·").

An understanding of the chemistry of Hg is important before attempting to make accurate analytical measurements:

It is accepted that Hg²⁺ reacts with atomic Hg as shown:

(1) Hg₂²⁺ = Hg²⁺ + :Hg⁰ K_eq1 = [Hg²⁺]/[Hg₂²⁺]

If the diatom contains two Hg⁺ atoms, then they would have a spin-free electron and be paramagnetic. Each of these electrons (5d¹⁰6s¹) is, in fact, diamagnetic, indicating no spin-free electron (no 6s¹). In addition to the lack of paramagnetic behavior, Raman spectroscopy has confirmed the existence of a covalent Hg-Hg bond. Consequently, the existence of monovalent Hg does not appear to be the case. This is critical to understanding the chemistry of aqueous Hg solutions.

The stability of the Hg₂²⁺ toward disproportionation, i.e., the K_eq1, can be calculated as follows, using the standard potentials found in any table of half-cell redox reactions:

(2) Hg₂²⁺ + 2e^- = 2Hg⁰ E = 0.789 v vs. N.H.E.

(3) Hg₂²⁺ = 2Hg²⁺ + 2e^- E = -0.920 v vs. N.H.E.

(4) = (2) + (3) 2Hg₂²⁺ + 2e^- = 2Hg²⁺ + 2Hg⁰ + 2e^- ΔE = -0.131 v vs. N.H.E.

Equation (4) is simply the sum of the half-cells (2) and (3), where (3) is written as an oxidation half-cell, thereby requiring a reverse in sign from positive 0.92 to -0.92 volts. Dividing through both sides by 2 and canceling out the electrons gives equation (1):

(1) Hg₂²⁺ = Hg²⁺ + Hg⁰ ΔE = -0.131 v vs. N.H.E.

K_eq1 = [Hg²⁺]/[Hg₂²⁺] (The activity of the elemental Hg⁰ is assumed to be = 1.)

The equilibrium constant (K_eq1) is calculated using fundamental thermodynamics as follows:

ΔG = -nFΔE = -RT ln K_eq1

Where:

• G = Gibbs free energy
• n = 1 (one gram equivalent of Hg₂²⁺ is converted to one gram atomic weight of Hg²⁺ and one gram atomic weight of Hg⁰)
• F = Faraday
• ΔE = -0.131 v vs. N.H.E.
• R = Rydberg constant
• T = temperature in degrees K

Substituting in values from above:

ΔE = RT/nF × 2.303 log K_eq1 = 0.0591 log K_eq1

-0.131 / 0.0591 = log K_eq1

-2.21 = log K_eq1

K_eq1 = [Hg²⁺]/[Hg₂²⁺] = 6.3 × 10^-3

This value of K_eq1 shows that Hg²⁺ (aq.) should react with Hg⁰ to form Hg₂²⁺ (aq.), which has been shown to be the case, as stated above. Below, it will be shown how this equilibrium is used to explain aqueous/acidic Hg chemistry.

Manufacturing Hg Aqueous/Acidic CRMs — Hg CRMs in aqueous nitric acid are manufactured at IV from the metal (Hg⁰) in a borosilicate glass reactor. The reaction is as follows:

(5) Hg⁰ + HNO₃ → Hg²⁺ + NO₂↑ (brown gas) + xs HNO₃ → heat to expel NO₂ → Hg²⁺ + xs HNO₃ → dilute with H₂O to make stock products/concentrates

Reaction (5) is heated until no more brown fumes are observed, and the solution appears water white. This Hg solution is assayed by two methods, namely EDTA titration and ICP. The concentration of Hg is also calculated from the weight of the metal and the weight of the final solution. When the "brown" fumes are not removed from the solution, the EDTA value is low relative to the ICP value, due to the reduction of the Hg²⁺ to Hg₂²⁺, which is shown as reaction (6) below:

(6) 2H₂O + 2NO₂ (dissolved/brown) + 2Hg²⁺ → Hg₂²⁺ + 2NO₃^- + 4H⁺

2Hg²⁺ + 2e^- = Hg₂²⁺ +0.920 v vs. N.H.E.

2[NO₃^- + 2H⁺ + e^- = NO₂ + H₂O] +0.80 v vs. N.H.E.

2H₂O + 2NO₂ (dissolved) + 2Hg²⁺ → Hg₂²⁺ + 2NO₃^- + 4H⁺ (ΔE = +0.12 v)

The positive ΔE shows that this reaction is thermodynamically favorable, i.e., the Gibbs free energy (ΔG = -nFΔE) is negative.

The Mercury (I) Ion — As stated in inorganic textbooks such as Cotton and Wilkinson, the mercurous Hg(I) ion exists in a binuclear state and is expressed in chemical reactions as Hg₂²⁺. The binuclear ion is not stable toward disproportionation, according to the following reaction (1), which was also discussed above:

(1) L + Hg₂²⁺ = HgL²⁺ + Hg⁰

Reaction (1) occurs when a complexing or precipitation agent (L) comes in contact with the divalent-diatomic ion, forming a complex or precipitate more readily with Hg²⁺ than with Hg₂²⁺ (which is most of the time). Chloride, which forms Hg₂Cl₂, is one of the few examples where disproportionation of the Hg₂²⁺ may not occur when it comes in contact with the chloride ion, but the extent is dependent upon HCl concentration and solution acidity.

QC — As discussed above, the bulk solution of Hg²⁺ (made from the metal — 20K, 10K, 1K) is titrated with EDTA and is assayed by ICP.

The EDTA (L^4-) titration:

(7) Hg²⁺ + L^4- = HgL^2- (Log K1 = 21.8)

If Hg₂²⁺ is present, the EDTA will react with the mercury (II) diatomic divalent ion in such a way that it will disproportionate to divalent mercury and elemental mercury, which is shown below as follows:

(8) Hg₂²⁺ + L^4- = HgL^2- + Hg⁰

The ICP assay:

The ICP assay value will not be in error, so long as it does not disproportionate, giving elemental Hg. Elemental Hg will give false high results by ICP, due to what we refer to as a vaporization interference. Therefore, if the diatomic divalent mercury ion has already disproportionated, then the Hg⁰ will be present, causing a high result for the ICP assay. This high ICP result, paired with a low result for the EDTA, can be used to determine if disproportionation has occurred or not. If the ICP assay is equal to the gravimetric value, then we know that the divalent diatomic ion is present but has not undergone disproportionation. If there is no difference between the assay values obtained from EDTA titrimetry and ICP, and both equal the gravimetric Hg value, then we know that the lot is free of the divalent diatomic ion.

Issues With Hg

Issues with Hg are very common. For example, our QC department experienced an interference issue where an unexpected impurity in a lot of trace-metals grade HCl reacted as follows:

(9) Hg₂²⁺ + ?L = HgL^+? + Hg⁰

The elemental Hg⁰ gives a vaporization interference with the ICP-OES measurement, making the intensity appear high (this is caused by the high vapor pressure of the elemental mercury, causing more to reach the plasma through vaporization than just from nebulization alone). The impurity was later identified as H₃PO₄. These are the types of unexpected problems we need to be prepared for when attempting to measure trace Hg by ICP.

The following are suggestions for avoiding problems with Hg:

• Avoid neutral/basic media, especially amines and alcohol amines.
• Avoid mixing with tartrate — it will be reduced to the metallic form.
• Reducing environments can convert Hg²⁺ to the Hg₂²⁺ dimer or the metallic form.
• Hg₂²⁺ dissociates into metallic mercury Hg⁰ and Hg²⁺ when a ligand is added.
• Hg²⁺ can reduce to Hg₂²⁺ if NO₂ gases

Detailed handling information related to Mercury containing solutions, as well as suggestions for ICP analyses of Mercury, may be found by clicking on the Hg element symbol on our interactive periodic table.